1. Field of the Invention
The present invention is related to handovers in a communication system and particularly, but not exclusively, to mobile telephone station handovers in an IP-based telecommunications network.
2. Description of the Related Art
Prior art office-based communications systems usually operate conventional fixed-line telephone units linked via an internal switchboard or PBX (private branch exchange). Such fixed-line systems are able to provide relatively high voice quality. However, user mobility is severely impaired.
The advent of digital mobile technologies such as GSM, however, means that mobile systems can now provide equivalent, if not higher, voice quality than fixed-line systems. Mobile systems also allow greater freedom of movement for the user within the office than do fixed-line systems.
WIO (Wireless Intranet Office) is a proprietary communications system developed by the applicants which introduces the concept of utilising mobile telephone units, such as conventional GSM mobile stations, in an office environment. The system makes use of a known concept called Internet Telephony or Voice-over-IP.
Voice-over-IP is a technology which allows sound, data and video information to be transmitted over existing IP-based Local or Wide Area Networks or the Internet. The technology thus provides for convergence and integration of three different media types over the same network.
Prior to the advent of Voice-over-IP, offices often operated three separate networks for the transmission of these media types. As indicated above, fixed-line telephone systems coupled to an in-house PBX provided for voice communication, an office-based LAN or Intranet (i.e. a packet-switched internal network), comprising computer terminals linked via network cards and under the control of a server station, provided for the transmission of “conventional” computer data and video cameras linked to monitors via fixed line or remote transmission link provided for video communication.
Voice-over-IP effectively combines these three media types such that they can be transmitted simultaneously on the same packet-switched network or IP-router throughout the office environment and beyond the confines of the office.
In order to provide for such media convergence, Voice-over-IP often uses a specific ITU (International Telecommunication Union) standard protocol to control the media flow over the Intranet. One common standard protocol used in Voice-over-IP systems, and the one used in the WIO system, is termed H.323.
H.323 is an ITU standard for multimedia communications (voice, video and data) and allows multimedia streaming over conventional packet-switched networks. The protocol provides for call control, multimedia management and bandwidth management for both point-to-point (2 end-users) and multipoint (3 or more end-users) conferences. H.323 also supports standard video and audio codecs (compression/decompression methods such as MPEG) and supports data sharing via the T.120 standard.
Furthermore, H.323 is network, platform and application independent allowing any H.323 compliant terminal to operate in conjunction with any other terminal.
The H.323 standard defines the use of three further command and control protocols:                a) H.245 for call control;        b) Q.931 based protocol for call signalling; and        c) The RAS (Registrations, Admissions and Status) signalling function.        
The H.245 control protocol is responsible for control messages governing the operation of the H.323 terminal including capability exchanges, commands and indications. Q.931 is used to set up a connection between two terminals. RAS governs registration, admission and bandwidth functions between endpoints and gatekeepers (defined later).
For a H.323 based communication system, the standard defines four major components:    1. Terminal    2. Gateway    3. Gatekeeper    4. Multipoint Control Unit (MCU).
Terminals are the user end-points on the network, for example a telephone or fax unit or a computer terminal. All H.323 compliant terminals must support voice communications, but video and data support is optional.
Gateways connect H.323 networks to other networks or protocols. For an entirely internal communications network, i.e. with no external call facility, gateways are not required.
Gatekeepers are the control centre of the Voice-over-IP network. It is under the control of a gatekeeper that most transactions (communication between two terminals) are established. Primary functions of the gatekeeper are address translation, bandwidth management and call control to limit the number of simultaneous H.323 connections and the total bandwidth used by those connections. An H.323 “zone” is defined as the collection of all terminals, gateways and multipoint-control units (MCU—defined below) which are managed by a single gatekeeper.
Multipoint Control Units (MCU) support communications between three or more terminals. The MCU comprises a multipoint controller (MC) which performs H.245 negotiations between all terminals to determine common audio and video processing capabilities, and a multipoint processor (MP) which routes audio, video and data streams between terminals.
A conventional Voice-over-IP system described hereinabove normally utilises standard fixed-line telephone systems which are subject to the disadvantages outlined above, namely the lack of mobility and the lack of user features.
The WIO system improves on the Voice-over-IP concept in that it provides for the use of conventional mobile telephone units, such as GSM mobile stations, within the Voice-over-IP system. To provide for such mobile communications within an office-based communication network, WIO combines known Voice-over-IP, as described above, with conventional GSM-based mobile systems.
Thus, internal office calls are routed only through the office intranet and external calls are routed through both the intranet and the GSM network. Such a system provides most or all of the features supported by the mobile stations and the GSM network such as telephone directories, short messaging, multiparty services, data calls, call barring, call forwarding etc. WIO, therefore, provides for integrated voice, video and data communications by interfacing an H.323-based voice-over-IP network with a GSM mobile network.
The WIO system is a cellular network, similar to the conventional GSM network and is divided into H.323 Zones, defined in more detail later. One H.323 Zone may comprise a number of cells.
Given the cellular nature of the WIO system, a major issue to be addressed is that of handovers. As a mobile station moves from cell to cell it reports its location to the BTS or equivalent controller. When it moves from one zone to another, a handover is needed to a different gateway. A similar consideration applies to mobile stations in the conventional GSM network.
In such conventional GSM systems, the need for a handover of a mobile station to a different cell of the network is normally determined by a number of parameters but predominantly including the mobile station measuring the strength of signals transmitted from several base transceiver stations.
Similarly, a mobile station operating in the WIO system is able to compare the signal strengths of the signals received by several base stations, in different cells, in the network.
However, added complexities arise in the WIO system since a mobile unit operating within such a system is not only able to move between cells within the system, but also between zones and even between the system itself and an external GSM network.
It can be seen, therefore, that there are several different types of handovers which may need to be executed in the normal operation of a WIO system. These types of handovers are:                a) The handover of a mobile from one WIO cell to another whilst engaged in a call.        b) The handover of a mobile from one WIO zone to another whilst engaged in a call.        c) The handover of a mobile from a cell within the WIO system to a cell within an external GSM system.        d) The handover from a cell within an external GSM system to a cell within the WIO system.        